There has been known a gas sensor which is provided in an exhaust passage of an internal combustion engine such as an automotive engine and which includes a detection element whose output changes with the concentration of a specific gas (oxygen, NOX, etc.) contained in exhaust gas. For example, the detection element of an NOX sensor capable of detecting the concentration of NOX has a layered structure including at least one cell composed of a solid electrolyte member and a pair of electrodes provided thereon. The detection element includes a first measurement chamber (first internal cavity) into which exhaust gas is introduced via a diffusion resistor section (first diffusion path); and a second measurement chamber (second internal cavity) into which exhaust gas whose oxygen has been pumped out in the first measurement chamber is introduced (for example, see Non-patent Document 1).
Oxygen contained in the exhaust gas introduced into the first measurement chamber is pumped out to the outside by the cell, whereby the concentration of oxygen remaining in the exhaust gas introduced into the second measurement chamber is adjusted to a predetermined low level. NOX contained in exhaust gas includes NO and NO2. According to Non-patent Document 1, most of NO2 is reduced to NO in the first measurement chamber. In the second measurement chamber, NO is decomposed to nitrogen and oxygen by the catalytic action of an electrode formed of noble metal such as Pt or Rh. At that time, oxygen derived from the decomposed NO (oxygen having constituted NO or NO2 (NOX)) is pumped out by the cell. In the cell, electrons conveyed via oxygen ions are detected in the form of current. The oxygen concentration detected in this manner is offset by the concentration of the residual oxygen (the above-mentioned adjusted oxygen concentration), whereby the concentration of NOX-originating oxygen (ultimately, the concentration of NOX) is detected.